What is the source of “unknown OID” errors in Rail

问题:

When replicating an app to production, my POSTGIS table columns started misbehaving, with Rails informing me there was an "unknown OID 26865" and that the fields would be treated as String.

Instead of current_pos yielding e. g. #<RGeo::Geographic::SphericalPointImpl:0x22fabdc "POINT (13.39318248760133 52.52908798020595)"> I would get 0101000020E6100000FFDD958664C92A403619DEE6B2434A40. It looked like the activerecord-postgis-adapter was not installed, or installed badly, but I eliminated that possibility by testing for the existence of data type RGeo::Feature::Point and by test-assigning
current_pos = "POINT (13.39318248760133 52.52908798020595)" to the field - which proceeded without error but then yielded another incomprehensible hex string like the above.

Also, strangely enough, POSTGIS was working correctly within the database, e.g. giving correct results for a ST_DISTANCE query. A very limited problem thus, where writing, writing-parsing (from Point to hex format), manipulating by SQL and reading all worked, only the parsing upon read didn't.

When I tried to use migrations to ensure the database column would have the correct type, the migrations failed, giving

undefined method `st_point' for #<ActiveRecord::ConnectionAdapters::PostgreSQL::TableDefinition:0x00000005cb80b8>

回答1:

I spent several hours trying all kinds of solutions, even re-installing the server from scratch, double-checking version numbers of everything, installing a slightly newer version of Ruby and a slightly older version of POSTGIS (to match my other environment), exporting the database and starting with a clean one, and so on. After I had done migrations and arrived at the "undefined method st_point" error, I was finally able to find the solution via Google, way down in a Github issue, and it's really simple:

In config/database.yml, swap out postgres:// for postgis:// in the database url. If you're using Heroku, this may require some ugly manipulation:

production:
  url: <%= ENV.fetch('DATABASE_URL', '').sub(/^postgres/, "postgis") %>

So silly...